DETERMINATION OF THE COMMENSURATELY MODULATED STRUCTURE OF NBGE2/5TE2 FROM TWINNED-CRYSTAL DATA

The commensurate modulation of the NbGe2/5Te2 structure has been determined from X-ray diffraction data collected on a twinned crystal. The (3 + 1)D superspace group is P2(1)/n(0betagamma), beta = 0.0, gamma = 0.4. The basic unit cell dimensions are a = 6.4437(9), b = 16.004(2), c = 3.8936(4) angstrom, alpha = 119.10(1)degrees, V = 350.85(7) angstrom3, and Z = 4. Final R factors are 0.051, 0.122, 0.187, and 0.110 for 742 main reflections, 1573 first-order superreflections, 927 second-order superreflections, and the combined set, respectively. Twinning is taken into account by an extra basic vector; the intensities of all reflections, overlapping as well as non-overlapping, are used in the final refinement. The twin volume ratio is refined to nu = 0.418(1). NbGe2/5Te2 is a layered compound consisting of [Te/Nb, Ge/Te] sandwiches with a close to orthorhombic basic structure modulated by strong monoclinic, commensurate occupational and displacive waves. Bond distances in the structure of NbGe2/5Te2 compare very well with those in the three- and seven-fold orthorhombic superstructures of NbGe1/3Te2 and NbGe3/7Te2, respectively. It is shown that all three structures are built from the same intrasandwich subunits, and that the stacking, and accordingly the 3D symmetry, is determined by the subunit geometry. The pinning of two sandwiches on each other is related to a few short intersandwich Te-Te contacts that in turn can be correlated to the electronic stabilization of the compound. (C) 1994 Academic Press, Inc.